Lightly crosslinked polyethylene polystyrene blend foams and process for making

ABSTRACT

A polymer foam made from a blend of a low density ethylenic polymer and styrenic polymer utilizing a volatile blowing agent and a free radical generating compound as a crosslinking agent. The free radical generating compound provides light crosslinking and increases the foamability of the polymer blend.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 06/697,653, filed Feb. 4,1985, now U.S. Pat. No. 4,605,682.

BACKGROUND OF THE INVENTION

This invention relates to polymer foam materials made from blends of anethylenic polymer and a styrenic polymer, and more particularly to afoamable blend of a polyethylene and a polystyrene lightly crosslinkedby free radical generating compounds.

Both polystyrene foams and polyolefin foams have heretofore been usedfor a variety of purposes. Polystyrene foams are lightweight and haveboth rigidity and good shape retention. However, polystyrene foams arenot resistant to solvents and are deficient in certain importantproperties such as flexibility, compressive recovery, and capacity toabsorb repeated impacts. Polyolefin foams, while having flexibility andgood impact absorbing properties, while generally suffered from lowthermal resistance and have been subject to creep. It would be desirableto blend the two resins to produce foams having the advantageousproperties of each.

Prior efforts to blend these two types of polymers have had to face thethreshold problem that olefin and styrenic polymers are generallyincompatible. For example, British Pat. No. 1,460,621 teaches a foamprepared from a blend polyethylene and polystyrene, combined with ablock copolymer of styrene and butadiene or a graft polymer of styrenemonomer onto polyethylene. The block or graft polymers are added torender the two other resins more compatible.

Japanese Patent Application No. 55/181,384 also teaches a foam which isa blend of polyethylene and polystyrene. However, the resin blend isfoamable only for certain specified ratios of melt indices ofpolystyrene to polyethylene, and for certain polystyrene to polyethyleneweight ratios.

Finally, Kannan et al, U.S. Pat. No. 4,262,052, each a foamed compositematerial, one component thereof being a blend of a polyolefin, a vinylaromatic polymer such as styrene, and a graft polymer formed by graftpolymerizing a vinyl aromatic monomer onto the backbone of thepolyolefin. A crosslinking agent such as an organic peroxide aids thegrafting reaction.

However, all of these previous efforts to blend polyolefin andpolystyrene resins to produce foams have either been complicated by theneed for a separate reaction sequence to form a block or graft copolymerto add to the blend or have been limited to certain melt index or weightratios of the starting resins. For example, the previously discussedJapanese Patent Application No. 55/181,384 utilized a very viscous, lowmelt index polyethylene. Such a viscous polymer causes pumping,pressure, and mixing problems in conventional melt processing equipmentsuch as extruders.

Accordingly, the need exists in the art for a composition blendingtogether these two types of resins so that they can be foamed readily byconventional melt processing techniques.

SUMMARY OF THE INVENTION

The present invention provides a polymeric foam composition made from ablend of about 95 to 50% by weight of a low density ethylenic polymerand about 5 to 50% by weight of a styrenic polymer utilizing freeradical generating compounds such as organic peroxides as crosslinkingagents. The blend also contains a volatile blowing agent.

Through the use of crosslinking by a free radical generating compoundgood quality foams are produced. Free radical generating compoundsuseful in the practice of the present invention are described in U.S.Pat. No. 3,646,155, the disclosure of which is hereby incorporated byreference. With respect to organic peroxide free radical generatingagents, it is believed that while the peroxide crosslinks the ethyleniccomponent of the blend, it depolymerizes the styrenic component,altering the viscosity ratio of the two components toward a more readilyfoamable direction. Surprisingly, not only branched chain low densityethylenic polymers but also linear low density ethylenic polymers can beused to produce foams in accordance with the present invention. Linearlow density olefins are generally difficult to foam by conventionalextrusion processes.

When made in accordance with the present invention, the foams have lowdensities, large cross-section, large cell size, reasonably low opencell content, good appearance, and good flow stability. Upon ambientaging, the foams exhibit excellent dimensional stability, even whenrelatively inexpensive blowing agents are utilized. For example, a lowdensity polyethylene foam made with inexpensive FC-12(dichlorodifluoromethane) blowing agent will shrink more than 30% of itsinitial volume during aging. However, a blend foam of the presentinvention made with the same blowing agent will shrink less then 20%,and depending on the particular polyolefin/polystyrene blend utilized,less than 10% of its initial volume.

Accordingly, it is an object of the present invention to provide afoamable composition and process of making it using a blend of a lowdensity ethylenic polymer and a styrenic polymer. This and other objectsand advantages of the invention will become apparent from the followingdetailed description and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process and composition of the present invention permit theproduction of low density foams by an extrusion process from either alinear or a branched-chain low density ethylenic polymer blended with astyrenic polymer. The invention provides a foam having desirableproperties of both ethylenic and styrenic foams. In the case where alinear low density ethylenic polymer is utilized in the blend, theresulting foam also exhibits the additional desirable properties ofhigher temperature resistance and a higher heat distortion temperaturethan comparable foams of branched-chain olefins.

The polymer blend of the present invention comprises from about 5 to 50%by weight of a styrenic polymer and from about 95 to 50% by weight of alow density ethylenic polymer. As the styrenic polymer, polystyrene ispreferred although other polymers or copolymers of vinyl aromaticmonomers such as methylstyrene, ethylstyrene, or halogenated styrenesmay be utilized. Exemplary copolymerizable monomers for the styreniccomponent of the blend also include acrylonitrile or butadiene. Thus,both crystal (or general purpose) polystyrene and high impactpolystyrene can be used in the present invention although crystalpolystyrene is usually used. As the low density ethylenic polymer, lowdensity polyethylene (LDPE) and linear low density polyethylene (LLDPE)are preferred, although other low density polyolefins or copolymersthereof may be utilized. Exemplary copolymerizable monomers for theethylenic component of the blend include ethylenically unsaturated acidsand esters such as acrylates and methacrylates, vinyl esters ofcarboxylic acids, as well as other olefinic hydrocarbons such aspropylene, butylene, and butene.

The terms "low density polyethylene" and "linear low densitypolyethylene" are used herein in their conventional sense. The former isobtained by a high pressure process and typically has a density of about0.910 to 0.932. LLDPE resins are available by either the high pressureor low pressure processing. Those used in the present invention areusually obtained through a Ziegler reaction and may be obtained ascopolymers with higher alpha olefins. Their density is not as welldefined as the LDPE's and may range from about 0.90 to 0.945.

The blowing agents useful in the practice of the present invention arewell known and may comprise solids which decompose into gaseous productsat extrusion temperatures or volatile liquids. A preferred class ofblowing agents is the group of halogenated hydrocarbon compounds havingfrom 1 to 4 carbon atoms and hydrocarbons having from 3 to 5 carbonatoms. Dichlorodifluoromethane (FC-12), 1,2 dichlorotetrafluoroethane(FC-114), trichloromonofluoromethane (FC-11), and mixtures thereof aremost preferred. When these halogenated hydrocarbon compounds are used asthe blowing agent, there can be from about 0.013 to 0.50 gram mole, andpreferably about 0.04 to 0.30 gram mole of such blowing agent per 100grams of polymer resin blend in the flowable gel or admixture. Mixturesof such blowing agents may also be used.

The blowing agent is compounded into the flowable polymer blend in themelt processing apparatus in proportions to make the desired degree ofexpansion in the resulting foamed cellular product to make productshaving foamed densities down to about 0.6 pounds per cubic foot (pcf).Depending on the amount of blowing agent added, the resulting foamedmaterials may have densities from about 0.6 to 15.0 pcf.

Crosslinking agents useful in the practice of the present invention arefree radical generating compounds such as the organic peroxides andperesters disclosed in U.S. Pat. No. 3,646,155, the disclosure of whichis hereby incorporated by reference. Specific examples of free radicalgenerating compounds include benzoyl peroxide; dichlorobenzoyl peroxide;di-tertiary butyl peroxide; 2,5 dimethyl-2,5 di(peroxybenzoate)hexyne-3; 1,3-bis(t-butyl peroxy isopropyl)benzene; lauroylperoxide; tertiary butyl peracetate; 2,5-dimethyl-2,5di(t-butylperoxy)hexane; and tertiary butyl perbenzoate, as well as azocompounds such as azobis-isobutyronitrile and dimethylazodiiosbutyrate.Most preferred as a crosslinking agent is dicumyl peroxide. Thecrosslinking agents are preferably added in an amount between about0.001 to 0.5 pph, and most preferably between 0.01 to 0.1 pph of polymerblend.

The organic peroxides are believed to perform the dual function ofcrosslinking the ethylenic polymer component of the blend whiledepolymerizing the styrenic component, thus altering the viscosities ofthe two polymer components toward a more foamable condition. Because acontrolled amount of crosslinking agent is added downstream in the meltprocessing equipment, no processing difficulties arise due to highpolymer viscosity. This controlled amount of crosslinking agent producesa lightly crosslinked polymer blend melt which remains flowable withoutsuffering severe melt fracture and has a melt index greater than about0.05 and preferably greater than about 0.1.

In accordance with the process of the present invention, foams of ablend of a low density ethylenic polymer and a styrenic polymer may bemade on conventional melt processing apparatus such as by continuousextrusion on a screw-type extruder. Such an extruder typically comprisesa series of sequential zones including a feed zone, compression and meltzone, metering zone, and mixing zone. The barrel of the extruder may beprovided with conventional electric heaters for zoned temperaturecontrol.

An inlet is provided for adding a mixture of fluid blowing agent andcrosslinking agent under pressure to the polymer blend in the extruderbarrel between the metering and mixing zones. Crosslinking agent ispumped, in a controllable manner, into the stream of fluid blowing agentupstream of the injection nozzle. The blowing agent and crosslinkingagent are compounded into the starting polymer in a conventional mannerto form a flowable gel or admixture, preferably in a continuous manner.Thus, the polymer blend, blowing agent, and crosslinking agent may becombined in the mixing zone of an extruder using heat to plastify thepolymer resin, pressure to maintain the blowing agent in a liquid state,and mechanical working to obtain thorough mixing.

The discharge end of the mixing zone of the extruder is connected,through a cooling and temperature control zone, to a die orifice. Thehot polymer gel is cooled and then passed through the die orifice into azone of lower pressure (e.g., normal ambient air atmosphere) where theblowing agent is activated and the polymer gel expands to a lowerdensity, cellular mass. As the foamed extrusion forms, it is conductedaway from the die and allowed to cool and harden.

In practice, the temperature of the feed zone is maintained at 140°±20°C., the temperature of the melting, metering, and mixing zones ismaintained at 180°±20° C., and the temperature in the cooling andtemperature control zone is maintained at 120°±20° C. The temperature ofthe polymer gel as it expands through the die orifice is preferably justabove the temperature at which solid polymer would crystallize out ofthe gel and will vary depending upon the particular polymer blendutilized.

As is conventional, finely divided solid materials such as talc, calciumsilicate, zine stearate, and the like can advantageously be incorporatedwith the polymer gel prior to expansion. Such finely divided materialsaid in controlling the size of the cells and may be employed in amountsup to five percent by weight of the polymer. Numerous fillers, pigments,lubricants, and the like well known in the art can also be incorporatedas desired.

The specific working examples that follow are intended to illustrate theinvention, but are not to be taken as limiting the scope thereof. In theexamples, parts and percentages are by weight unless otherwise specifiedor required by the context.

EXAMPLE I

The apparatus used in this example is a 11/4" screw type extruder havingtwo additional zones for mixing and cooling at the end of usualsequential zones for feeding, melting and metering. An opening forblowing agent injection is provided on the extruder barrel between themetering and mixing zones. A small syringe-type pump is connected to theblowing agent stream for additive injections. At the end of coolingzone, there is attached a die orifice having an opening of rectangularshape. The height of the opening, called die gap hereinafter, isadjustable while its width is fixed at 6.25 mm (0.25").

An 80/20 by weight blend of a low density polyethylene (2.3 melt index;0.923 gm/cc density) and a polystyrene of 200,000 weight averagemolecular weight was used. The two granular polymers were blended in atumbler. A small amount of (0.05 pph) of talc was adhered to the polymergranules by use of a wetting agent. Dichlorodifluoromethane (FC-12) wasused as the blowing agent. Dicumyl peroxide was used as the crosslinkingagent and was injected with the blowing agent in the amounts indicatedin Table I. Extruder temperatures were set at 140° C. at feeding zone,180° C. at melting and metering zone, an 200° C. at mixing zone,respectively. The gel temperature was maintained at 111° C. The testresults are summarized below in Table I.

As can be seen, the blend, which was outside the operable ranges setforth in Japanese Patent Application No. 55/181,384, was indeedinoperable. Total foam collapse resulted. The foam improved dramaticallywhen a small amount (0.0051 pph) of organic peroxide crosslinking agentwas introduced. At a higher level of peroxide (0.081 pph), the foamquality was even better.

The foams produced had low densities, large cross-section, large cellsize, a reasonably low percentage of open cells, and had a goodappearance and good flow stability. Advantageously, the line pressuresdid not rise appreciably. The foams showed excellent dimensionalstability (shrinkage less than 11%) during ambient aging. This comparesquite favorably with low density polyethylene foams made with the sameblowing agent which typically shrink over 30% of their initial volumeduring ambient aging.

                                      TABLE I                                     __________________________________________________________________________              DICUMYL                                                                  FC-12                                                                              PEROXIDE                                                                              GEL  DIE                                                         LEVEL                                                                              LEVEL   TEMP.                                                                              GAP DIE   EXTRUD.                                      TEST (pph)                                                                              (pph)   (°C.)                                                                       (mm)                                                                              PRESS.                                                                              PRESS.                                       No.  (1)  (2)     (3)  (4) (5)   (6)                                          __________________________________________________________________________    1    19.6 --      111  1.78                                                                              36.9  77.3                                                                1.52                                                                              44.3  78.7                                                                1.27                                                                              54.8  100.6                                        2    19.6 0.051   111  2.29                                                                              37.3  73.8                                                                2.03                                                                              40.1  77.3                                                                1.78                                                                              42.2  84.4                                         3    19.6 0.081   111  2.54                                                                              33.7  66.8                                                                2.29                                                                              34.8  75.9                                                                2.03                                                                              35.2  70.3                                                                1.78                                                                              36.6  77.3                                                                1.52                                                                              38.0  82.3                                         __________________________________________________________________________        FOAM FOAM       CELL OPEN                                                     THICK                                                                              WIDTH                                                                              FOAM  SIZE CELL                                                                              FOAM  FOAM                                       TEST                                                                              (mm) (mm) DENSITY                                                                             (mm) (%) APPEAR.                                                                             STAB.                                      NO. (7)  (8)  (9)   (10) (11)                                                                              (12)  (13)                                       __________________________________________________________________________    1    6.1 14.5 253.0 ND   ND  C     --                                              5.6 14.0 288.0 ND   ND  C     --                                              4.3 11.4 363.0 ND   ND  C     --                                         2   22.9 33.8 28.5  1.25 53.0                                                                              V,H   E                                              20.6 33.5 34.3  1.16 61.8                                                                              V,H   E                                              18.3 33.0 37.2  1.25 72.0                                                                              V,H   W                                          3   27.4 37.8 27.5  1.81 33.5                                                                              V,H   W                                              25.4 37.1 26.4  1.25 29.3                                                                              V,H   G                                              25.4 36.3 26.4  1.62 34.7                                                                              G     E                                              22.1 36.8 25.3  1.16 33.6                                                                              G     W                                              18.0 34.5 27.9  1.01 41.3                                                                              G     W                                          __________________________________________________________________________     Notes: All formulations contained 0.05 pph talc                               ND not determined                                                             (1) difluorodichloromethane mixed in per hundred parts of polymer             (2) parts of dicumyl peroxide mixed in per hundred parts of polymer           (3) temperature in degrees centigrade to which the gel was cooled down        prior to foam expansion                                                       (4) gap of die opening in millimeters at which samples were taken             (5), (6) pressure in kilograms per square centimeter at the die and at th     extruder discharge, respectively                                              (7), (7) thickness and width of foam body in millimeters measured within      about five minutes after extrusion                                            (9) density of foam body in kilograms per cubic meter measured in about       one month                                                                     (10) cell size in millimeter in horizontal direction determined per ASTM      D3576                                                                         (11) open cell content in percent determined per ASTM D2856-A                 (12) subjective judgment of foam appearance; C = total collapse, V = void     of smaller dimensions contained in the foam, H = hairy skin originating       from prefoaming, G = good, E = excellent                                      (13) the behavior of extrudate; W = wobbly strand, G = good, E = excellen                                                                              

EXAMPLE II

The apparatus and operating conditions of Example I were again utilized.The polymer used in this example was an 80/20 by weight blend of alinear low desnity polyethylene (melt index 1.0; 0.935 gm/cc density)and a polystyrene of 200,000 weight average molecular weight. Theaddition of 0.02 pph peroxide crosslinking agent improved the flowstability and appearance of the foam. The results are reported in TableII below.

                                      TABLE II                                    __________________________________________________________________________              DICUMYL                                                                  FC-12                                                                              PEROXIDE                                                                              GEL  DIE                                                         LEVEL                                                                              LEVEL   TEMP.                                                                              GAP DIE   EXTRUD.                                      TEST (pph)                                                                              (pph)   (°C.)                                                                       (mm)                                                                              PRESS.                                                                              PRESS.                                       NO   (1)  (2)     (3)  (4) (5)   (6)                                          __________________________________________________________________________    1    19.6 0       124  2.03                                                                              29.5  88.6                                                                1.78                                                                              30.2  91.4                                                                1.52                                                                              31.6  94.9                                         2    19.6 0.02    124  2.03                                                                              30.6  98.4                                                                1.78                                                                              33.7  101.9                                                               1.52                                                                              36.9  105.4                                                               1.27                                                                              42.5  107.2                                        __________________________________________________________________________        FOAM FOAM       CELL OPEN                                                     THICK                                                                              WIDTH                                                                              FOAM  SIZE CELL                                                                              FOAM  FOAM                                       TEST                                                                              (mm) (mm) DENSITY                                                                             (mm) (%) APPEAR.                                                                             STAB.                                      NO. (7)  (8)  (9)   (10) (11)                                                                              (12)  (13)                                       __________________________________________________________________________    1   12.7 27.2 35.5  0.38 97.1                                                                              B,O   W                                              10.7 26.9 33.6  0.39 95.1                                                                              O     W                                              10.2 27.2 37.2  0.34 95.4                                                                              O     G                                          2   14.5 27.7 30.9  0.10 86.8                                                                              B,O   G                                              11.7 28.7 33.3  0.23 96.2                                                                              O     G                                              11.7 28.4 34.1  0.11 94.3                                                                              O     E                                              11.4 27.9 34.6  0.10 96.1                                                                              O     E                                          __________________________________________________________________________     Notes: all formulations contained 0.05 pph talc                               (1) difluorodichloromethane mixed in per hundred parts of polymer             (2) parts of dicumyl peroxide mixed in per hundred parts of polymer           (3) temperature in degrees centigrade to which the gel was cooled down        prior to foam expansion                                                       (4) gap of die opening in millimeters at which samples were taken             (5),(6) pressure in kilograms per square centimeter at the die and at the     extruder discharge, respectively                                              (7),(8) thickness and width of foam body in millimeters measured within       about five minutes after extrusion                                            (9) density of foam body in kilograms per cubic meter measured in about       one month                                                                     (10) cell size in millimeter in horizontal direction determined per ASTM      D3576                                                                         (11) open cell content in percent determined per ASTM D2856-A                 (12) subjective judgment of foam appearance; O = open cells, B = blips or     large voids contained in the foam, G = good, E = excellent                    (13) the behavior of extrudate; W = wobbly strand, G = good, E = excellen                                                                              

While the methods and compositions herein described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to these precise methods and compositions, and thatchanges may be made in either without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. An expandable composition of a lightlycrosslinked blend of a styrene polymer and a low density ethylenepolymer comprising: from 5 to 50% by weight of a styrene polymer; from95 to 50% by weight of an ethylene polymer; a sufficient amount of afree radical generating compound as a crosslinking agent; and a volatileblowing agent.
 2. The composition of claim 1 in which said free radicalgenerating compound is an organic peroxide.
 3. The composition of claim2 in which said organic peroxide is present in an amount of from about0.01 to 0.1 pph by weight.
 4. The composition of claim 2 in which saidorganic peroxide is dicumyl peroxide.
 5. The composition of claim 1 inwhich said ethylene polymer is a low density polyethylene.
 6. Thecomposition of claim 1 in which said styrene polymer is polystyrene. 7.A lightly crosslinked polymer foam material produced by the processof:(a) melt processing under pressure from about 95 to 50% by weight ofa low density ethylene polymer and from about 5 to 50% by weight of astyrene polymer together with a volatile blowing agent and a sufficientamount of a free radical generating compound as a crosslinking agent, toform a flowable admixture; and (b) extruding said admixture through adie to a zone of lower pressure and activating said blowing agent toexpand said admixture of polymers to a cellular structure crosslinkedwith itself.